Helicopter main rotor lift and rotor driving torque produce reaction forces and moments on the helicopter main gearbox. In addition to the primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor system. These vibratory loads produce vibrations and noise within the aircraft that are extremely discomforting and fatiguing to the passengers.
One vibratory load that is of particular concern results from the interaction of the rotor blades with blade vortices developed by the preceding blades during rotation. As the rotor blade rotates, the air flows passing over and under the blade combine downstream from the trailing edge creating a vortex. During normal flight modes, the blade vortices do not cause any particular problem. However, in certain instances, such as when the aircraft is descending, the trailing blade contacts the blade vortex generating an impulsive noise or slap. This contact with the vortex also creates a vibration within the rotor system that transfers into the cabin. These vibrations can be upwards of 5/rev (i.e., 5 times per revolution of the rotor system). The noise and vibrations generated by the blade interaction with the vortices results in passenger and crew discomfort.
Blade vortex interaction also generates an external noise signature which can be easily detected at long range, increasing the aircraft's vulnerability when in a hostile environment. With the increasing use of helicopters for night reconnaissance missions, it is desirable to minimize the external noise signature of the aircraft.
Many attempts have been made over the years to alleviate or reduce blade vortex interactions. A considerable amount of those attempts have been directed toward passive type systems wherein the blade is designed to weaken the vortex at the blade tip. See, for example, U.S. Pat. No. 4,324,530 which discloses a rotor blade with an anhedral swept tapered tip which reduces the intensity and shifts the location of the tip trailing edge vortex so as to reduce the occurrence of blade vortex interactions.
While passive solutions have provided some reduction in blade vortex interaction, those solutions also tend to negatively impact the flight characteristics of the rotor blade.
Active rotor control systems have recently been proposed to counteract blade vortex interactions. These systems are typically designed to change the motion of the rotor blade to miss the blade vortex or cut the vortex differently so as to reduce contact with the blade vortex. One of these systems is called higher harmonic blade pitch control wherein the blade pitch is controlled to reduce the vortex at the blade tip. While the reduced blade tip vortex does lead to lower noise from blade vortex interaction, the change in blade pitch also reduces the aerodynamic characteristics for the entire blade.
Another active control system is discussed in U.S. Pat. No. 5,588,800. This active control system is mounted within a helicopter rotor blade and includes actuatable flaps on the rotor that are controlled to reduce the blade vortex interaction. An actuator is used to control the movement of the flaps and can be either mechanical, electrical, pneumatic, or hydraulic. U.S. Pat. No. 5,639,215 discloses a similar actuatable flap assembly. In this assembly, the actuator is a mechanical actuator that is either a push-rod type device, a linkage, or a servo-motor driven rack.
Although the prior art systems for actively controlling the rotor blade interactions with the blade vortex are empirically better than the passive systems described above, these prior art systems do not address the realistic problems associated with mounting an actuation system within a rotor blade to control the flaps in the desired manner.
A need, therefore, exists for an improved actuation system for use in an active rotor control system to control flaps on a rotor blade for improving the blade's aerodynamic performance while reducing noise generating by blade vortex interactions.